Process for producing articles made from polyurethane foam and additive for performing this process

ABSTRACT

A process for producing polyurethane foam articles is described, in which the article is produced in the conventional manner by foaming in a mould and a release agent is applied to the mould surface prior to filling with the foam-producing components. According to the invention, a solvent-free, aqueous release agent is used in combination with one or more alkali metal and/or ammonium salts of higher fatty acids and one or more higher alcohols and the combination is applied in the hot state, preferably at a temperature of at least 75° C. to the mould surface. In addition, an additive is described, which is suitable for performing the process according to the invention and contains one or more alkali metal and/or ammonium salts of higher fatty acids, one or more higher alcohols and otherwise water. The process according to the invention has the advantage that it not only avoids the use of environmentally prejudicial organic solvents, but also leads to excellent release agent films, from which the water is very largely evaporated within a very short time and namely within the very short cycle times of modern foam plants.

The invention relates to a process for producing polyurethane foamarticles, in which the article is produced in a conventional manner byfoaming in a mould and prior to filling with the foam-producingcomponents, the mould is treated with a release agent.

Polyurethane foam articles are produced in moulds and this can takeplace according to the most varied prior art processes. A large numberof possible variants, which are known to the Expert, exist with respectto the foam-producing components to be used and with respect to theequipment and moulds to be used. Generally the mould is filled either inthe open or closed state through a feed channel using the polyol andisocyanate components mixed by means of a mixing head. Apart from thesought product and its desired characteristics, the composition of thecomponents depends on whether working is to take place according to theone-shot process (direct process) or according to the two-shot process(prepolymer or semiprepolymer process). All these different possibleprocedures are described in the literature, so that there is no need fora detailed discussion thereof. For example, reference is made to"Integralschaumstoffe", Piechota and Rohr, 1975; Kunststoff-Handbuch,vol. VII, "Polyurethane", 1966; "Schaumkunststoffe", published by theFachverband Schaumkunststoffe in GKV, 1976 and Wittfoht,Kunststoff-Technisches Worterbuch, part 3.

In order to permit an easy, clean removal of the foam articlescompletely polymerized in the metal, wood or plastic moulds, the mouldsurfaces coming into contact with the foam-producing components or thepolyurethane foam are treated with release or parting agents (mouldrelease agents) prior to the filling of the mould. For this purpose, themould surfaces are given a uniform coating of a release agent, therebyproducing a release film. This conventionally takes place by knownspraying and sprinkling processes, the release agent being atomized bythe high material pressure or by additional compressive forces. Apartfrom good mould release characteristics, in special cases the releaseagent must also exert an influence with regards to the surfaceproperties of the finished polyurethane foam article. Thus, specificadditives, e.g. in the cold flexible foam field (seats, headrests,armrests, etc. in cars) produce the open-cell structure and consequentlyenable the foam article to "breath" in the necessary way.

Conventional release agents consist of organic solvents andrelease-active substances dissolved, dispersed, suspended or emulsifiedtherein and which are referred to as solids. As is known, they cancontain waxes, greases, fatas, silicone compounds, fatty acid amides,fatty acid esters, plasticizers, stabilizers, accelerators, etc. Themain solvents present in the release agent, such as e.g. frigen,methylenchloride, thichloroethane, perchloroethylene, gasoline andhigh-boiling gasoline hydrocarbons are used as carriers, so that therelease-active substances can be applied as a uniform release film tothe mould surfaces.

The solids composition, solids proportion and solvent combinations arefixed as a function of the mould temperature, the air-exposure time(time between release agent application and component introduction) andthe particular foam system (integral skin, rigid integral skin or coldflexible foam). In this way it is possible to produce the most variedarticles with widely differing characteristics. Thus, e.g. integral skinfoam is used for producing steering wheels for cars, specialrequirements being made on the surface (uniformity and good grippingcharacteristics). When producing integral skin foam, the mouldtemperature is generally between 30° and 50° C. However, the mouldtemperature in the RIM process (RIM foam), frequently used for producingpolished mouldings is 50° to 70° C. Similar temperatures, namely in therange 40° to 70° C., are exhibited by the moulds in producing articlesfrom cold flexible foam, such as seats, headrests and armrests for cars.The mould temperature is generally 30° to 50° C. when producing articlesfrom rigid foam, such as brackets for cars, window shutters,refrigerator components, furniture parts and chairs.

On introducing the release agent, which generally takes place by meansof a spraying or sprinkling apparatus, the solvents largely evaporate asan azeotropic mixture from the mould surface heated to between 25° and70° C., as a function of the foam system present. Together with theoverspray, these vapours are removed by means of correspondingexhausting units from the working area. Quite apart from the loss ofvaluable raw materials, such pollutant emissions are highly prejudicialto the environment. Recycling processes with adsorption plants would beconveivable, but in this case the solid present in finely divided form,which cannot be eliminated either by filters or water walls constitute aserious problem. The absorption coatings immediately covered with waxand fat would have to be regenerated at very short time intervals, whichwould lead to very considerable expenditure and effort.

As a result of the aforementioned problems, attempts have been made forsome time to use water-based release agents, i.e. release agents inwhich the organic solvents are largely and preferably completelyreplaced by water. Low solvent or solvent-free release agents are known.They are essentially fat, wax or soap emulsions or dispersions. Theproblem exists in connection with these release agents that followingrelease agent application, in most of the aforementioned foam systems,the mould can only be filled with the foam-producing components when thesolvent, in this case water, has largely evaporated, so that thepolymerization process is not prejudiced.

The reason why inadequate evaporation of the water when using aqueousrelease agents leads to lower quality products is the competing reactionbetween water and isocyanate groups (R--N═C═O+H₂ O→R--NH₂ +CO₂) runningparallel to the polyol-isocyanate reaction, leading to the liberation ofcarbon dioxide and a partial displacement of the precisely matchedpolyol-isocyanate ratio. This leads to foam disturbances in the mouldranging from discolourations, bubbles, voids (blisters) to partial foamcollapse. Since due to the system-based circulation cycles of modernfoam plants generally only 10 to 50 seconds at temperatures of max. 65°C. are available for evaporating the release agent films,which has beenprovided inadequate for a sufficient elimination of the water from themore or less strongly water-combining release film, aqueous releaseagents cannot be used in most of the existing foam technologies.

It is known to use aqueous release agents for the almost completelysuperseded hot foaming, where water evaporation is brought about by highenergy-consuming mould temperatures. In addition, aqueous release agentsare used where only limited demands are made on the surface quality ofthe foam articles produced, because their surfaces are not visible. Anexample of this is back-foaming.

In order to be able to economically use aqueous release agents in otherfields, approximately 10 to 30% evaporation accelerators have been addedthereto for reducing the airing times. These consist of low-boilingalcohols, ketones, esters, etc. which apart from the possibleinterfering reactions with the isocyanate, also are a burden on theenvironment. The use of such part aqueous, part solvent-containingsystems has not as yet proved successful in practice.

Thus, the problem of the invention is, whilst obviating thedisadvantages linked with the poor evaporation behavior of the water, topermit the economic and environmentally non-prejudicial use of aqueousrelease agents, also in those foam systems, where this has not hithertobeen possible as a result of the short cycle times present in today'smoulding systems and plants.

According to the invention this problem is solved by a process forproducing polyurethane foam articles of the aforementioned type, whereina solvent-free, aqueous release agent is used In combination with one ormore alkali metal and/or ammonium salts of higher C₁₈ -C₃₀ -fatty acidsand one or more higher C₄ -C₂₆ -alcohols and the combination is appliedto the mould surface in the hot state.

It has surprisingly been found that the process according to theinvention leads to a drastic reduction in the airing time of the releasefilms applied and to a very large removal of the water from the releaseagent film. This is all the more surprising in that the soap emulsionsknown per se as release agents lead to an aqueous micella formationduring the evaporation process. It is also surprising that thewater-combining pseudocrystalline structures occurring when usingconventional emulsifiers are apparently eliminated by the procedureaccording to the invention. The combination used according to theinvention is in the hot state a highly fluid release agent, which formsa uniform release agent film from which the water is largely evaporatedwithin the aforementioned short cycle times of industrial foam plants.

There are no limitations with regards to the aqueous release agentsusable in the process according to the invention and which arecompletely or at least substantially free from organic solvents, i.e.they are present in the form of emulsions or dispersions and containconventional release-active substances and solids (see hereinbefore),which are incorporated into the aqueous base. This takes place inaccordance with the conventional processes of emulsion and dispersiontechnology (cf. e.g. Seifen-Ole-Fette-Wachse, vol. 107, No. 14, 1981, pp391 to 402). Generally the water or at least part thereof is introducedfirst and then the different release-active substances and optionallyconventional additives (see hereinafter) are incorporated into thesesubstances and if necessary emulsifiers are used. The sequence ofincorporating the release-active substances can be chosen at random. Itis obviously possible to use elevated temperatures for facilitating theemulsification process. No further details need be provided in thisconnection, because the Expert is able to fix the composition of therelease agent in accordance with the particular requirements of the caseand choose the in each case most favourable procedure for producing sucha release agent.

Particularly suitable for the process according to the invention areaqueous release agents, which contain as the release-active substancesynthetic and/or natural waxes, such as microwax and in particularpolyethylene wax. The choice of the in each case suitable wax fallswithin the routine activity of the Expert and is dependent on theintended use. Thus, e.g. for cold flexbile foam softer (lower melting)waxes are used than when producing articles from integral skin foam.

As stated hereinbefore, the aqueous release agents used according to theinvention can contain further conventional components. These includeinter alia biocides, polyurethane foam stabilizers matched to the systemsuch as e.g. silicone glycols or polysiloxane glycols and accelerators,such as the tin accelerators (e.g. dibutyl tin dilaurate) and amineaccelerators (e.g. bis-(2-dimethylaminoetheyl)-ether) used in the priorart solvent systems. In order to obtain the open-cell structure oftenrequired in cold flexible foam (see hereinbefore), it is possible e.g.to incorporate commercially available glycolsiloxane surfactants(silicone - glycol copolymers). These water-soluble surfactants can alsobe directly added to the polyol component.

The additive which according to the invention is combined with theaqueous release agent consists of an aqueous solution of one or morealkali metal salts, preferably potassium salts, of higher C₁₈ -C₃₀ andpreferably C₂₂ -C₂₆ -fatty acids and one or more higher C₄ -C₂₆ andpreferably C₆ -C₁₀ -alcohols. The additive is prepared by introducingthe alkali metal salt or salts of the higher fatty acid or acids or thefatty acids and lye, or ammonia or amines (see above) as well as thehigher alcohol or alcohols into water and heating the mixture toapproximately 80° C., accompanied by stirring. The higher alcoholcontent of thus prepared additive is 1 to 12% by weight. The content ofalkali metal salt of higher fatty acid is usualy in the range of 1 to20% by weight and preferably 5 to 9% by weight. The additive is added inan adequate quantity and normally a quantity of 10 to 50% by weight tothe aqueous release agent used according to the invention, as a functionof its characteristics and its intended use.

Alternatively the components of the aforementioned additive, namely thealkali metal and/or ammonium salt or salts of higher fatty acids and thehigher alcohol or alcohols can be added directly to the aqueous releaseagent. This preferably takes plce directly prior to the use of therelease agent in order to avoid double heating. This procedure is lesspreferred than the use of the previously described additive.

The aqueous release agent and the additive combined therewith accordingto the invention are brought together accompanied by thorough mixing andare usually heated in a continuous heater to the desired temperature ofat least 75° C. and preferably at least 80° C. The heated combination ispermanently kept circulating, in order to ensure a uniform thoroughmixing and avoid a separation of any components. The combination heatedto the desired temperature is then applied in a conventional manner tothe mould surface, which preferably takes place by spraying (seehereinbefore).

It is important that the concentration of the higher alcohol or alcoholsin the combination used according to the invention is no greater than 6%by weight and preferably no greater than 4% by weight. Correspondinglythe concentration is usually in the range 1 to 6% by weight. Theconcentration of the alkali metal and/or ammonium salt or salts ofhigher fatty acids, based on the total combination, is generally in therange 0.5 to 10% by weight and preferably 2 to 7% by weight.

As stated hereinbefore, the use of the inventive combination or aqueousrelease agent and alkali metal and/or ammonium salts of higher fattyacids, as well as higher alcohols leads to a drastic reduction of theevaporation times of the release agent films applied and to a very highdegree of elimination of the water from the release agent film. Thiseffect only occurs if the aqueous release agent mixed with the additiveis heated to at least 75° C. prior to application. Apart from thesurface tension reduction, this effect is probably due to thedestruction of the aqueous micella occurring when using conventionalsoap emulsions, as well as to the removal of water-combiningpseudocrystalline structures, which are caused by the use ofconventional emulsifiers. In addition, a destruction of the emulsionswhich are otherwise usually unstable at these temperatures is avoidedthrough the stabilizing action of the additive used according to theinvention. The inventive additive has in itself release-activecharacteristics and consequently additionally improves the releaseeffectiveness of the conventional release emulsions.

When producing articles from cold flexible foam (car parts) very goodresults have been obtained with the following aqueous release agent inaccordance with the inventive process:

Polyethylene wax: 0.5% by weight

Emulsifier (secondary linear alkane sulphonate in the form of the sodiumsalt; 60% effective substance): 1.3% by weight

Foam stabilizer (see hereinbefore): 0.3% by weight

Water: Remainder

This aqueous release agent was combined with different quantities (e.g.in a ratio of 3:1) of the additive composed in the hereinafter indicatedform:

Behenic acid: 8% by weight

KOH: 1.6% by weight

C₆ -C₁₀ -alcohol: 4% by weight

Water: Remainder

We claim:
 1. A process for producing polyurethane foam articles in amould, comprising applying to the mould a release agent compositioncomprising (a) an aqueous emulsion or dispersion of natural or syntheticwax, (b) an alkali metal or ammonium salt of a C₁₈ -C₃₀ -fatty acid and(c) a C₄ -C₂₆ -alcohol.
 2. A process according to claim 1 wherein (c) isC₄ -C₁₂ alcohol.
 3. A process according to claim 1 wherein saidcomposition comprises no more than about 6%, by weight, of the component(c).
 4. A process according to claim 1 wherein said compositioncomprises no more than about 10%, by weight, of component (b).
 5. Aprocess according to claim 1 wherein the release agent is applied at atemperature of at least 75° C.
 6. A process according to claim 1 whereinthe release agent is applied at temperature of at least 80° C.
 7. Aprocess according to claim 1 wherein component (b) comprises an alkalimetal salt of a C₂₂ -C₂₆ -fatty acid.
 8. A process according to claim 1wherein the alkali metal is potassium.
 9. A process according to claim 1wherein component (b) is potassium behenate.
 10. A process according toclaim 1 wherein component (c) is a C₆ -C₁₀ -alcohol.
 11. A processaccording to claim 1 wherein said wax comprises polyethylene wax.